Courier-IMAP is a server that provides IMAP access to Maildirs
mailboxes. This IMAP server does NOT handle traditional mailbox files
(/var/spool/mail, and derivatives), it was written for the specific
purpose of providing IMAP access to Maildirs.

The Courier-IMAP implements some extensions to the basic Maildir mail
storage format in order to support some additional features, such as
folders and "soft" quotas. This is done in a way that's backwards
compatible with the standard Maildir users (although, of course, they
will ignore the additional features). Other software that understands
and uses the same Maildir extensions is SqWebMail, and maildrop.

"Maildir" is a directory-based mail storage format originally introduced
in the Qmail mail server, and adopted as an alternative mail storage
format by both Exim and Postfix. Courier-IMAP is popular on
Qmail/Exim/Postfix sites that are configured to use maildirs.  The
primary advantage of maildirs is that multiple applications can access
the same maildir simultaneously without requiring any kind of locking
whatsoever. It is a faster and more efficient way to store mail. It
works particularly well over NFS, which has a long history of
locking-related woes.

It is part of the Courier mail server (http://www.courier-mta.org) but
can be configured as a standalone IMAP server.

Features include an extremely small memory footprint, abstract
authentication modules (e.g. /etc/passwd, LDAP, PAM), virtual mailbox
support, resource controls, SSL/TLS, and IPv6 support.
